delay remove_file logic to free_inode
solve type conversion warnings
1. Make inode number starts from 1 rather than 0

   internal mapping from inode number n to real index n-1 occurs at macro IBLOCK (i-1)

2. Make the start index of data block independent of IBLOCK

   use IBLOCK(INODE_NUM, BLOCK_NUM) + 1 to mark the start index

3. Restore the stupid bug coming from the upstream

   IBLOCK *+3* makes block 10 to be wasted, should be *+2*! The *only* side effect of this bug is that block 10 will be wasted!
Make sure alloc_inode complains once the inode is exhausted
Print warnings when file size exceeds the limit
Make sure alloc_block complains exactly when the data block is exhausted